1. Field of Invention
The present invention provides a process for producing abrasive particles, the abrasive particles produced according to the process, and a process for removing a film layer using a CMP slurry containing particles made by the process.
2. Description of Related Art
Chemical-mechanical polishing (CMP) slurries are used, for example, to planarize surfaces during the fabrication of semiconductor chips and related electronic components. CMP slurries typically include reactive chemical agents and abrasive particles dispersed in a liquid carrier. The abrasive particles perform a grinding function when pressed against the surface being polished using a polishing pad, and separately, the reactive chemical agents serve to oxidize the surface.
It is well known that the size, composition, and morphology of the abrasive particles used in a CMP slurry can have a profound effect on the polishing rate and surface finishing. Over the years, CMP slurries have been formulated using abrasive particles formed using, for example, alumina (Al2O3), cerium oxide, or ceria (CeO2), iron oxide (Fe2O3), silica (SiO2), silicon carbide (SiC), silicon nitride (Si3N4), tin oxide (SnO2), titania (TiO2), titanium carbide (TiC), tungsten oxide (WO3), yttria (Y2O3), zirconia (ZrO2), and combinations thereof.
Known abrasive particles for use in CMP slurries include colloidal silica, which is produced by condensation in aqueous solution. Another is fumed silica, which may be produced by a continuous flame hydrolysis technique involving the conversion of silicon tetrachloride (SiCl4) to the gas phase using an oxy-hydrogen flame. Fumed silica is by far the most widely used abrasive particle.
Calcination is another method of producing abrasive particles for use in CMP slurries. During the calcination process, precursors such as carbonates, oxalates, nitrates, and sulfates, are converted into their corresponding oxides at very high temperature. After the calcination process is complete, the resulting oxides must be milled to obtain proper particle sizes and distributions to provide desired removal rates and to prevent scratching.
The calcination process, although widely used, does present certain disadvantages. For example, it tends to be energy intensive and can produce toxic and/or corrosive gaseous byproducts. In addition, contaminants are easily introduced during the calcination and subsequent milling processes. Finally, it is difficult to obtain a narrow particle size distribution.
The basic mechanism of the CMP process is the simultaneous formation of a removable surface layer, such as via oxidation of a metal surface or via hydrolysis of an oxide or nitride layer, coupled with the mechanical removal of the removable surface layer using abrasive particles pressed between a work piece and a polishing pad that are in motion relative to one another. In CMP slurries for removing copper films, the mechanical (abrasive) effect and oxidizing function are separately provided by the different components. That is, abrasive particles mainly contribute the mechanical effect, while chemical oxidizing agents give rise to a chemical (redox) reaction.
Numerous chemical additives exist to improve film removal rates, to adjust the selectivity of removal rates between various materials, and to allow better surface finishing and fewer defects. Hydrogen peroxide, ferric nitrate, potassium iodate and periodate are widely used as oxidizing chemicals in copper CMP slurries, to improve removal rates relative to slurries having only abrasive particles. Most CMP slurries are formed by combining two separate components, namely: (1) abrasive particles dispersed in a liquid medium; and (2) chemical additives (e.g., a chemical oxidizer). The separate components are mixed together immediately prior to use and, once blended, have a shelf life of typically only about 5 days or less. The chemical oxidizer in conventional CMP slurries tends to lose its oxidative efficacy if it remains unused for long periods.
While the use of chemical oxidizers improves the metal removal rate to industrially practicable levels, the chemical oxidizers in the slurry continue to oxidize metal until they are expended or removed. Hence, chemical oxidizers are one of main contributors to the problem of dishing or pitting of metal surfaces, which results from continued oxidative attack on an already planar metal surface, even in the absence of abrasive particles.